1. Field of the Invention
The present invention relates to a small antenna device in which a pair of radiating slots are provided to constitute a diversity antenna.
2. Description of the Related Art
As the related art of such an antenna device, as shown in FIG. 6, a configuration is known in which a pair of radiating slots 32 and 33 open in a metal case 31 of a wireless LAN card 30 and are electromagnetically coupled to microstrip lines 34 and 35 to be fed, respectively (for example, see Japanese Unexamined Patent Application Publication No. 2003-234615 (pages 3-4 and FIG. 1)). Each of the radiating slots 32 and 33 opens in an L shape to extend along the external shape of the metal case 31. Since one end of the radiating slot 32 faces one end of the radiating slot 33, the pair of radiating slots 32 and 33 are line-symmetrically arranged. The microstrip lines 34 and 35 are formed in a circuit board 36 housed in the metal case 31 so as to be connected to a power feeding circuit (not shown). The microstrip line 34 faces a power feeding position of the radiating slot 32 and the microstrip line 35 faces a power feeding position of the radiating slot 33.
In the related art antenna device having such a schematic configuration, when the radiating slots 32 and 33 are excited by power feeding via the microstrip lines 34 and 35, a radiation electric field is generated in each of the radiating slots 32 and 33, and then an electric wave is generated. At that time, the polarization direction of the electric wave generated by the radiating slot 32 and the polarization direction of the electric wave generated by the radiating slot 33 are different from each other. Accordingly, if a diversity antenna has the pair of radiating slots 32 and 33, a wireless LAN signal wave in which a variation in the polarization direction occurs due to multipath may be received.
If the pair of radiating slots 32 and 33 are provided in parallel and excited in the metal case 31 which serves as a common conductor member, surface currents flowing the environs of the individual radiating slots 32 and 33 are intensively coupled to each other, and thus an isolation characteristic tends to deteriorate. For this reason, the pair of radiating slots are significantly spaced from each other in order to enhance the isolation characteristic. If the gap between the radiating slots 32 and 33 is widened, the entire antenna device is made large. Accordingly, a desired reduction in size can be realized at the expense of the isolation characteristic to some extent. Further, in the related art, each of the radiating slots 32 and 33 is formed in the L shape along the external shape of the metal case 31 in view of a space factor. In this case, however, at the time of excitation, the electric field generated in each of the radiating slots 32 and 33 wraps around the side surface of the metal case 31, such that the lateral radiation is increased. Accordingly, the radiation electric fields of the individual radiating slots are intensively coupled to each other, which results in deterioration of the isolation characteristic. That is, when it is going to promote the entire device to be reduced in size, the isolation characteristic deteriorates, which tends to cause a trouble in antenna performance. To the contrary, when it is going to insure a desired isolation characteristic, there is a problem in that it is impossible to promote the entire device to be reduced in size.
Moreover, in such an antenna device, if the isolation characteristic is not favorable, in a transmission mode, radiation efficiency gets worse, and also, in a reception mode, a desired beam pattern cannot be formed. Accordingly, transmission or reception performance is degraded.